An album is a collection of audio recordings issued as a collection on compact disc (CD), vinyl, audio tape, or another medium. Albums of recorded music were developed in the early 20th century as individual 78-rpm records collected in a bound book resembling a photograph album; this format evolved after 1948 into single vinyl LP records played at 33 1⁄3 rpm. Vinyl LPs are still issued, though album sales in the 21st-century have mostly focused on CD and MP3 formats.